It can get a bit complex to sort out by first appearance of the RIC in the historical reference result. So alternatively, like to suggest taking a look at

{ {protocol}}{ {host}}{ {api}}Search/HistoricalSearch

{
  "Request": {
    "Identifier": "KYG211351195",
    "IdentifierType": "Isin",
    "Range": {
      "Start": "1995-01-01T00:00:00.000Z",
      "End": "2017-01-01T00:00:00.000Z"
    }
  }
}
   

we get back multiple entries such as

 {
            "Identifier": "0048.HK",
            "IdentifierType": "Ric",
            "Source": "",
            "Key": "VjF8MHgzMDAwMDAwMDAwMDAwMDAwfDB4MzAwMDAwMDAwMDAwMDAwMHx8fHx8fHwwMDQ4LkhLfA",
            "Description": "Historical Instrument",
            "InstrumentType": "Unknown",
            "Status": "Valid",
            "DomainCode": "6",
            "FirstDate": "1996-01-01T00:00:00.000Z",
            "LastDate": "2017-11-06T00:00:00.000Z",
            "History": []
        },
  

To make sense, we concentrate only on DomainCode=6, MarketPrice.

FirstDate and LastDate are the dates when the sinstrument was first valid and last valid,

with the exception of the instrument which is still valid, then LastDate will be the request date.

Does this request work better for your use case?

I think a good way is HistoricalReferenceExtractionRequest.

Please refer to REST API Tutorial 10 - Historical Reference

Similar to this:

{
    "ExtractionRequest": {
        "@odata.type": "#ThomsonReuters.Dss.Api.Extractions.ExtractionRequests.HistoricalReferenceExtractionRequest",
        "ContentFieldNames": [
           "RIC", "CUSIP", "ISIN", "SEDOL", "Issuer OrgID", "Exchange Code", "Currency Code", "Change Date"
        ],
        "IdentifierList": {
            "@odata.type": "#ThomsonReuters.Dss.Api.Extractions.ExtractionRequests.InstrumentIdentifierList",
            "InstrumentIdentifiers": [
                {
                    "Identifier": "DEA0V6ZA.D",
                    "IdentifierType": "Ric"
                },
                {
                    "Identifier": "0048.HK",
                    "IdentifierType": "Ric"
                },
                {
                    "Identifier": "00E1.SG",
                    "IdentifierType": "Ric"
                },
                {
                    "Identifier": "00E2.D",
                    "IdentifierType": "Ric"
                }
            ],
            "ValidationOptions": {"AllowHistoricalInstruments": true},
            "UseUserPreferencesForValidationOptions": false
        },
        "Condition": {
            "StartDate": "1996-05-01T00:00:00.000Z",
            "EndDate": "2017-05-31T23:59:59.000Z"
        }
    }
}

Depending on the use case. Let's take an example of RIC

0048.HK -> 0048.HK^A12-> 8321.HK

Once we have run the previous request, we know the common Isin = KYG211351195

We can issue

  {
    "ExtractionRequest": {
        "@odata.type": "#ThomsonReuters.Dss.Api.Extractions.ExtractionRequests.HistoricalReferenceExtractionRequest",
        "ContentFieldNames": [
           "RIC", "CUSIP", "ISIN", "SEDOL", "Issuer OrgID", "Exchange Code", "Currency Code", "Change Date"
        ],
        "IdentifierList": {
            "@odata.type": "#ThomsonReuters.Dss.Api.Extractions.ExtractionRequests.InstrumentIdentifierList",
            "InstrumentIdentifiers": [
                {
                    "Identifier": "KYG211351195",
                    "IdentifierType": "Isin"
                }
            ],
            "ValidationOptions": {"AllowHistoricalInstruments": true},
            "UseUserPreferencesForValidationOptions": false
        },
        "Condition": {
         "StartDate": "1990-05-01T00:00:00.000Z",
            "EndDate": "2017-05-31T23:59:59.000Z"
        }
    }
}

The received data will contain multiple change dates with RICs across this ISIN, with increasing change date.

The first set of changes carries the original RIC 0048.HK. The first occurrence of

0048.HK^A12 is when it was first valid. Then we have multiple changes of

0048.HK^A12 and the last set of changes contains RIC 8321.HK, the first one being when it was first valid.

Do you see how Historical Reference can be used to derive the RICs?